Chen Y. Wang

A new bonding technology using gold and tin multilayer composite structures

A bonding technology which utilizes chromium, gold, and tin and gold deposited directly on the backside of a device die to form a multilayer composite is reported. The substrate accepting the die is coated with chromium and gold layers. The die and the substrate are brought into contact and heated to 310-320 degrees C. Due to the unique feature of the gold-tin alloy system, the tin layer melts first and dissolves the gold layers of the composite to produce a solution mixed with solid, which in turn would dissolve a portion of the gold layer on the substrate to develop a near eutectic bonding. In the composite, since the tin layer is protected by an outer gold layer in the same vacuum cycle, tin oxidation, which is a major cause of difficulty in achieving quality bondings, is reduced. This technology thus eliminates the requirement of preforms, prevents tin oxidation, and provides precise control of the bonding thickness. Results of bonding 4-mm by 4-mm GaAs dice on alumina substrates show that high-quality bondings are obtained as determined by a scanning acoustic microscope (SAM). >

AuSn alloy phase diagram and properties related to its use as a bonding medium

Abstract AuSn eutectic alloy has been successfully used in microelectronic packaging for high reliability applications where a hard solder as well as a low processing temperature are required. A new multilayer bonding technology not only has produced nearly perfect bonding but also has reduced the processing temperature even below the eutectic melting point. Knowledge of the different phases of the alloy and their formation, as well as the interdiffusion that occurs, thus becomes important in studying the bonding principle and the long-term reliability. In this paper, we review a large number of publications on the AuSn system and summarize the important properties. We hope that this summary would further enhance the development of new AuSn bonding methods as a result of an overall understanding of oxidation and diffusion properties.

Au-In bonding below the eutectic temperature

A bonding method using Au-In alloy which requires a low process temperature of 200 degrees C to produce high temperature (454 degrees C) bonds is reported. Multiple layers of Au and In are deposited on semiconductor wafers in one vacuum cycle to reduce In oxidation. The semiconductor dice are then bonded to substrates coated with Au. Above 157 degrees C, the indium layer melts and dissolves the Au layers to form a mixture of liquid and solid. The solid-liquid interdiffusion process continues until the mixture solidifies to form the Au-In bond. A scanning acoustic microscope (SAM) was used to determine the excellent bonding quality before and after thermal shock tests while an energy dispersive X-ray (EDX) was employed to determine the composition of the resulting bonds. The resulting bond has an unbonding temperature greater than 545 degrees C. Due to the low process temperature, the stress on the bonded structure caused by thermal expansion mismatch is reduced. This type of bonding is useful when bonding at a low temperature is followed by a subsequent higher temperature process. >

Void free bonding of large silicon dice using gold-tin alloys

The successful bonding of large (6 mm*10 mm) silicon dice on alumina substrates with Au-Sn eutectic using a particular bonding technique is described. The bonding quality was examined by a scanning acoustic microscope having a resolution of 25 mu m and it was found that nearly perfect bondings have been achieved. Use of Au-Sn alloy rather than Au-Si resulted in not only lower processing temperature but also lower stress on the dice. This is confirmed by simple stress analysis. The die-bonded specimens endured 40 cycles of thermal shock between -196 degrees C (liquid nitrogen) and +160 degrees C (boiling cyclohexanol) without cracking or bond degradation despite the significant mismatch of thermal expansion coefficients between silicon and alumina. Storage tests at -196 and 250 degrees C also do not induce cracking or bond degradation. Pull test results indicate that the bondings are stronger than the silicon dice themselves. >

A low temperature bonding process using deposited gold-tin composites

Abstract Conventional bonding techniques using gold-tin preforms or paste usually require a process temperature near 320 °C to ensure complete melting of the preforms. However, a 320 °C process temperature is too high for many devices such as AlGaAs/GaAs and GaInAsP/InP laser diode chips. We report a technique which needs only 260 °C to produce nearly eutectic AuSn bonding. This technique utilizes the unique property of the gold-tin alloy system in that the 232°C tin melting point is significantly lower than the 280 °C eutectic point and solid state interdiffusion. The bonding medium consists of AuSn multilayer composite deposited directly on the object to be bonded. This technology eliminates the requirement of preforms, inhibits tin oxidation and provides good control of bonding layer thickness. Results of bonding of 3 mm x 5 mm GaAs dice show that high quality bondings are obtained as determined by a scanning acoustic microscope. The specimens underwent 40 cycles of thermal shock test between -196 °C and 160 °C without bonding degradation and die cracking. Scanning electron microscopy and energy-dispersive X-ray studies reveal interesting mechanisms of the bonding process. This new process is particularly useful for bonding electronic and optical devices which cannot take a temperature above 260 °C.

Temperature solution of five-layer structure with a circular embedded source and its applications

The temperature solution of a five-layer structure with a circular embedded source is reported. The solution is in the form of a single integration rather than the double integration of the solution for a structure with a rectangular source. An algorithm is developed for effective and accurate calculation of the integral solution. Based upon the solution and algorithm, a software program, PAMICE, has been written. Using the circular source solution, the CPU time for temperature calculation is reduced by a factor of 10 as compared to the rectangular source solution. An important application for the solution is the use of a circular source instead of square source as the unit source to produce the unit thermal profile for the real-time thermal design of integrated circuits. The solution is also valuable for the thermal study and analysis of devices having circular sources, such as power transistors, light emitting diodes, and laser diodes. Two application examples are presented. >

A bonding technique for thin GaAs dice with via holes using gold-tin composites

100 mu m thin 2-mm-by-3-mm GaAs dice with via holes have been successfully bonded on alumina substrates using the technique of gold-tin multilayer composite. The bondings are near perfect, as confirmed by a scanning acoustic microscope. Scanning electron microscope images of cross sections reveal that the bonding layers are very uniform and the thickness is 1.5 mu m. EDX analysis shows that the composition of the bonding layer is nearly gold-tin eutectic. No die crackings were observed after the bonding. The specimens underwent 40 cycles of thermal shock test between -196 degrees C and 160 degrees C without incurring bonding degradation and die cracking. >

Extremely reliable bonding of large silicon dice using gold-tin alloy

Large silicon dice have been bonded on alumina substrates with Au-Sn eutectic using a novel bonding technique. The bonding quality was examined by a scanning acoustic microscope (SAM) having a resolution of 20 mu m. Nearly perfect bondings have been achieved. Au-Sn alloy was chosen rather than the commonly used Au-Si alloy because the lower eutectic point of the Au-Sn system would result not only in lower processing temperature but also lower stress on the dice. The die-bonded specimens endured 40 cycles of thermal shock between -196 degrees C and +160 degrees C without cracking or bond degradation despite the significant mismatch of thermal expansion coefficients between silicon and alumina. Storage tests at -196 degrees C and +250 degrees C also do not induce cracking or bond degradation. Pull-test results indicate that the bondings are stronger than the silicon dice themselves.<<ETX>>

An eutectic bonding technology at a temperature below the eutectic point

The authors report a technique which needs only 240 degrees C to produce nearly eutectic Au-Sn bonding. After bonding, the device can take a postprocessing temperature of 250 degrees C without bonding degradation. The bonding medium consists of Au-Sn multilayer composite deposited directly on the object to be bonded. This technology also eliminates the preforms, reduces tin oxidation, and provides good control of bonding layer thickness. Results of bonding 2 mm*3 mm GaAs dice show that high-quality bondings are obtained, as evaluated by a scanning acoustic microscope. The specimens underwent 40 cycles of thermal shock test between -196 degrees C and 160 degrees C without bonding degradation and die cracking. Scanning electron microscopy and energy-dispersive X-ray studies reveal interesting characteristics of the bonding process. Melting point tests confirm that the bonding layer indeed has a melting temperature of 280 degrees C, higher than the 240 degrees C process temperature.<<ETX>>

Directly deposited fluxless lead-indium-gold composite solder

Lead-indium-gold multilayer composite solder has been developed for bonding electronic devices without the use of flux. The composite is deposited directly on GaAs wafers in high vacuum to inhibit indium oxidation. The gold layer on the composite further protects the indium layer from oxidation in atmosphere. Using the composite solder without flux, GaAs dies have been successfully bonded to alumina substrates at a process temperature of 250 degrees C. Nearly perfect joints are achieved as verified by a scanning acoustic microscope (SAM). Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy results indicate that the joint consists of AuIn/sub 2/ grains embedded in an In-Pb solid solution phase, as predicted from the Au-In-Pb phase diagram. Thermal shock as well as shear tests confirm that high quality bonding is obtained with the lead-indium-gold composite. >